The present invention relates to a dielectric ceramic composition having a resistance to reduction and to a multilayer ceramic capacitor or other electronic device using the dielectric ceramic composition.
A multilayer ceramic capacitor, a kind of electronic devices, is broadly used as a compact, large-capacity, high reliability electronic device. The number of capacitors used in each piece of electronic equipments has also increased. In recent years, along with increasing miniaturization and improving performance of equipments, there has been increasingly stronger demand for further reductions in size, increases in capacity, reductions in price and improvements in reliability in multilayer ceramic capacitors.
Multilayer ceramic capacitors are normally produced by stacking a paste for the internal electrode layers and a paste for the dielectric layers using the sheet method or printing method, etc. and then co-firing the internal electrode layers and dielectric layers in the stack together.
As the electroconductive material for the internal electrode layers, generally Pd or Pd alloys are used, but since Pd is high in price, relatively inexpensive Ni, Ni alloys, and other base metals have come into use. When using a base metal as the electroconductive material of the internal electrode layers, firing in the air oxidizes the internal electrode layers, therefore the co-firing of the dielectric layers and internal electrode layers has to be done in a reducing atmosphere. When being fired in a reducing atmosphere, however, the dielectric layers end up being reduced and becoming lower in resistivity. Therefore, non-reducing type of dielectric materials is being developed.
In multilayer ceramic capacitors using a nonreducing dielectric ceramic composition, insulation resistence (IR) remarkably deteriorates when an electric field is applied, more specifically, there is a disadvantage of short IR lifetime, or low reliabity.
There also arises a disadvantage that, when the dielectric composition is exposed to the direct-current electric field, a specific permittivity xcex5r declines over time. Also, a direct-current voltage can be superimposed on a capacitor and there is a disadvantage that when a direct-current voltage is applied to a capacitor having a dielectric composition wherein a strong dielectric composition is a main composition, a capacitance generally declines (DC bias characteristics). When a dielectric composition layer is made thinner in order to make a chip capacitor more compact and larger in capacitance, an electric field affecting the dielectric composition layer at the time of applying a direct-current voltage becomes strong, so the change of permittivity xcex5r over time, that is, the capacitance change over time becomes remarkably large and DC bias characteristics decline.
Further, a capacitor is also required to have excellent temperature characteristics. In particular, in some applications, it is desired that the temperature characteristics be smooth under harsh conditions. In recent years, multilayer ceramic capacitors have come into use for various types of electronic equipments such as the engine electronic control units (ECU), crank angle sensors, antilock brake system (ABS) modules, etc., mounted in engine compartments of automobiles. These types of electronic equipment are used for stabilizing engine control, drive control, and brake control, therefore they are required to have excellent circuit temperature stability.
These types of electronic equipment are used in the environment in which the temperature falls to as low as xe2x88x9220xc2x0 C. in the winter in cold areas or the temperature rises to as high as+130xc2x0 C. in the summer while an engine is working. Recently, there has been a trend toward reduction of the number of wire harnesses used for connecting electronic apparatuses and the controlled equipment. Electronic apparatuses are also being mounted outside of the vehicles in some cases. Therefore, the environment is becoming increasingly severe for electronic apparatuses. Accordingly, capacitors used for these electronic apparatuses have to have smooth temperature characteristics over a broad temperature range.
Temperature-compensating capacitor materials superior in temperature characteristics such as, (Sr, Ca)(Ti, Zr)O3 based, Ca(Ti, Zr) O3 based, Nd2O3-2TiO2 based, La2O3-2TiO2 based, and other materials are generally known, but these compositions have extremely low specific permittivities (generally 100 or less), so it is substantially impossible to produce a capacitor having large capacity.
To create dielectric ceramic compositions having the high permittivity and smooth capacitance-temperature characteristics, compositions comprised of BaTiO3 as a main component plus Nb2O5xe2x80x94Co3O4, MgOxe2x80x94Y, rare earth elements (Dy, Ho, etc.), Bi2O3xe2x80x94TiO2, etc. are known. A mechanism of smoothing the capacitance-temperature characteristic is not completely disclosed, but the Japanese Examined Patent Publication (Kokoku) No. 7-118431 proposes the way of smoothing the capacitance-temperature characteristic by dissolving Mg and rare earth elements inside a core-shell structure. However, in an article xe2x80x9cKey Engineering Materials Vols. 17 to 24, 157 to 158 (1999); A study on Capacitance Aging in Ni-Electroded, BaTiO2xe2x80x94Based MLCCs with X7R Characteristicsxe2x80x9d, it is reported that the core-shell structure is not essential to satisfy the X7R characteristic of the EIA Standards (xe2x88x9255 to 125xc2x0 C., xcex94C/C=xc2x115% or less).
Also, when looking at the temperature characteristics of a dielectric ceramic composition comprising BaTiO3 as a main component, because of the Curie temperature of pure BaTiO3 close to 130xc2x0 C., it is extremely difficult to satisfy the R characteristic of the capacitance-temperature characteristic (xcex94C/C=xc2x115% or less) in the region higher than 130xc2x0 C. Therefore, a BaTiO3 based high permittivity material can only satisfy the X7R characteristic of the EIA standard (xe2x88x9255 to 125xc2x0 C., AC/C=xc2x115% or less). Satisfaction of the X7R characteristic is not good enough to be used in an electronic apparatus of an automobile which is used in the above-mentioned harsh environments. The above electronic apparatus requires a dielectric ceramic composition satisfying the X8R characteristic of the EIA standard (xe2x88x9255 to 150xc2x0 C., xcex94C/C=+15% or less).
To satisfy the X8R characteristic in a dielectric ceramic composition comprised of BaTiO3 as a main component, it has been proposed to have the Curie temperature of the composition shift to the high temperature side by replacing the Ba in the BaTiO3 with Bi, Pb, etc. (Japanese Unexamined Patent Publication (Kokai) No. 10-25157 and No. 9-40465). Further, it has also been proposed to satisfy the X8R characteristic by selecting a BaTiO3+CaZrO3+ZnO+Nb2O5 based composition (Japanese Unexamined Patent Publication (Kokai) No. 4-295048, No. 4-292458, No. 4-292459, No. 5-109319, and No. 6-243721).
In each of these compositions, however, Pb, Bi, and Zn which are easily vaporized and scattered are used, so firing in the air or another oxidizing atmosphere is a prerequisite. Therefore, there are the problems that it is not possible to use an inexpensive base metal such as Ni for the internal electrodes of the capacitor and it is necessary to use Pd, Au, Ag, or other high priced precious metals.
On the other hand, to enable to attain the high permittivity, to satisfy the X8R characteristic and to be fired in a reducing atmosphere, the present inventors have already proposed a dielectric ceramic composition described below (The Japanese Unexamined Patent Publication (Kokai) No. 2000-154057). The dielectric ceramic composition at least comprises BaTiO3 as a main component, a first subcomponent including at least one compound selected from MgO, CaO, BaO, SrO and Cr2O3, a second component expressed by (Ba, Ca)x Sio2+x(note that x=0.8 to 1.2), a third subcomponent including at least one compound selected from V2O5, MoO3 and WO3 and a fourth subcomponent including an oxide of R1 (note that R1 is at least one element selected from Sc, Er, Tm, Yb and Lu). The ratios of the subcomponents to 100 moles of the main component are the first subcomponent: 0.1 to 3 moles, the second subcomponent: 2 to 10 moles, the third subcomponent: 0.01 to 0.5 moles and the fourth subcomponent: 0.5 to 7 moles (note that the number of moles of the fourth subcomponent is the ratio of R1 alone).
Also, the present inventors have proposed a dielectric ceramic composition described below recently (The Japanese Unexamined Patent Publication (Kokai) No. 2000-226862). The dielectric ceramic composition described in the filed specification comprises at least a main component including barium titanate, a first subcomponent including at least one compound selected from MgO, CaO, BaO, SrO and Cr2O3, a second subcomponent including silicon oxide, a third subcomponent including at least one compound selected from V2O5, MoO3 and WO3, a fourth subcomponent including an oxide of R1 (note that R1 is at least one element selected from Sc, Er, Tm, Yb and Lu), and a fifth subcomponent including CaZrO3 or CaO+ZrO2. The ratios of the subcomponents to 100 moles of the main component are the first subcomponent: 0.1 to 3 moles, the second subcomponent: 2 to 10 moles, the third subcomponent: 0.01 to 0.5 mole, the fourth subcomponent: 0.5 to 7 moles (note that the number of moles of the fourth subcomponent is the ratio of R1 alone), and the fifth subcomponent: 0 less than the fifth subcomponentxe2x89xa65 moles.
In any of the above inventions filed by the present inventors, the ratio of the first subcomponent such as MgO to 100 moles of the main component is not less than 0.1 mole.
The dielectric ceramic compositions of the above inventions filed by the present inventors are surely possible to attain the high permittivity, to satisfy the X8R characteristic, and to fire in a reducing atmosphere.
In the dielectric ceramic compositions of the above inventions, however, it was proved by the present inventors that when making a dielectric layer further thinner, it was difficult for the capacitance-temperature characteristic to satisfy the X8R characteristic and the insulation resistance lifetime was liable to decline. As to the capacitance-temperature characteristic, the capacitance change rate particularly on the high temperature side tends to increase, which is desired to be improved.
Also, among rare earth oxides, those including lanthanoids are high in price, so inexpensive substitutional elements capable of giving the same properties have been searched.
Furthermore, tendencies of higher integration and higher density of circuits have become increasingly stronger in recent years, and as a result, demands for compact and large capacitance capacitors have been increased. It has been demanded to make the dielectric layers inside further thinner.
An object of the present invention is to provide a dielectric ceramic composition having a high permittivity, capable of maintaining an insulation resistance lifetime, having a capacitance-temperature characteristic satisfying the X8R characteristic of the EIA standard (xe2x88x9255 to 150xc2x0 C., xcex94C/C=xc2x115% or less), able to be fired in a reducing atmosphere. Another object of the present invention is to provide a multilayer ceramic capacitor and other electronic devices using the dielectric ceramic composition, capable of realizing more compact body and larger capacitance, particularly with thinner layers.
To attain the above object, a dielectric ceramic composition according to the present invention comprises:
a main component including barium titanate,
a first subcomponent including an oxide of AE (note that AE is at least one element selected from Mg, Ca, Ba and Sr), and a second subcomponent including an oxide of R (note that R is at least one element selected from Y, Dy, Ho and Er),
wherein ratios of the subcomponents to 100 moles of the main component are
the first subcomponent: 0 mole less than the first subcomponent less than 0.1 mole, and the second subcomponent: 1 mole less than the second subcomponent less than 7 moles.
Preferably, ratios of the subcomponents to 100 moles of the main component are, the first subcomponent: 0.01 mole less than the first subcomponent less than 0.1 mole, and the second subcomponent: 1 mole less than the second subcomponentxe2x89xa66 moles.
Preferably, the ratio of the number of moles of the second subcomponent to the number of moles of the first subcomponent (the second subcomponent/the first subcomponent) is 10 less than (the second subcomponent/the first subcomponent) less than 500.
Preferably, the dielectric ceramic composition further comprises a sixth subcomponent including CaZrO3 or CaO+ZrO2, and a ratio of the sixth subcomponent to 100 moles of the main component is 0 mole less than the sixth subcomponent less than 5 moles.
Preferably, the dielectric ceramic composition further comprises a third subcomponent including MxSiO3 (note that M is at least one element selected from Ba, Ca, Sr, Li and B, x=1 when M=Ba, x=1 when M=Ca, X=1 when M=Sr, X=2 when M=Li, and X=⅔ when M=B), and a ratio of the third subcomponent to 100 moles of the main component is 2 molesxe2x89xa6the third subcomponentxe2x89xa610 moles.
Preferably, the dielectric ceramic composition further comprises a fourth subcomponent including at least one compound of MnO and Cr2O3, and a ratio of the fourth subcomponent to 100 moles of the main component is 0 mole less than the fourth subcomponentxe2x89xa60.5 mole.
Preferably, the dielectric ceramic composition further comprises a fifth subcomponent including at least one compound selected from V2O5, MoO3 and WO3, and a ratio of the fifth subcomponent to 100 moles of the main component is 0.01 molexe2x89xa6the fifth subcomponentxe2x89xa60.5 mole.
An electronic device according to the present invention is not particularly limited as far as it is an electronic device having a dielectric layer. An example is a multilayer ceramic capacitor element having a capacitor element body wherein dielectric layers and internal electrode layers are alternately stacked. In the present invention, the dielectric layer is comprised of any of the above dielectric ceramic compositions. A conductive material contained in the internal electrode layers is not particularly limited, for example, Ni or Ni alloy. In the present invention, the effect is large particularly when a thickness of the dielectric layer is less than about 10 xcexcm.
Operation and Effect of the Invention
The dielectric ceramic composition according to the present invention has a high specific permittivity and a capacitance-temperature characteristic satisfying the X8R characteristic of the EIA standard. Therefore, the ceramic chip capacitor and other electronic devices using the dielectric ceramic composition of the present invention can be preferably used in an environment of being exposed to a high temperature such as an engine room of vehicles.
Also, the dielectric ceramic composition according to the present invention does not include elements which vaporize and scatter, such as Pb, Bi and Zn. Therefore, firing in a reducing atmosphere is possible.
Namely, according to the present invention, it is possible to provide a dielectric ceramic composition having a high specific permittivity, capable of maintaining an insulation resistance lifetime, having a capacitance-temperature characteristic satisfying the X8R characteristic of the EIA standard, and able to be fired in a reducing atmosphere.
When producing an electronic device such as a ceramic chip capacitor by using the dielectric ceramic composition of the present invention, Ni, Ni alloy and other base metals can be used as an internal electrode, so a low-cost electronic device can be realized. Moreover, even if the dielectric ceramic composition is fired in a reducing atmosphere, an electronic device to be obtained satisfies the X8R characteristic, wherein a capacitance aging characteristic under an application of a direct current electric field is preferable (namely, a change of the capacitance over time is small), a decline of an insulation resistance is small and reliability is high.
Namely, since an electronic device, such as a multilayer ceramic capacitor, having dielectric layers composed of the dielectric ceramic composition of the present invention can provide a stable operation in a variety of apparatuses used under severe environments, such as an electronic apparatus of vehicles, reliability of the apparatuses to which the electronic device is applied can be remarkably improved.
As explained above, the dielectric composition of the present invention can be expected to give an effect also as a method of suppressing a decline of a temperature change rate in a high temperature range caused by making the dielectric layers thinner.
Furthermore, the dielectric ceramic composition according to the present invention has a long insulation resistance lifetime, and moreover, it has a stable DC bias characteristic (dependence on direct current voltage application of the permittivity) and a TC bias characteristic (a capacitance-temperature characteristic on applying a direct current voltage).
Also, since the dielectric ceramic composition according to the present invention does not comprise any hazardous substances, such as Pb and Bi, adverse effects on an environment due to disposal and processing after use are small.
Accordingly, by using the dielectric ceramic composition of the present invention, multilayer ceramic capacitors and other electronic devices having excellent characteristics can be easily provided. Also, by using the dielectric ceramic composition according to the present invention, the X8R characteristic can be satisfied and a decline of an insulation resistance lifetime can be effectively prevented even if dielectric layers are made thinner. Thus, multilayer ceramic capacitors and other electronic devices having a more compact body and a larger capacitance can be realized, particularly by making dielectric layers further thinner. As a result, mounting those devices on highly integrated circuits can become easier.
In a dielectric ceramic composition of the related art, there was a tendency that a capacitance-temperature characteristic particularly on the high temperature side declines as the dielectric layer becomes thinner. Namely, the curve of the change rate of the capacitance-temperature at a high temperature side tends to approach to the clockwise direction. On the other hand, according to the present invention, it is possible to make the curve of the capacitance-temperature change rate on the high temperature side approach to the anticlockwise direction. When applying the phenomenon to an electronic device satisfying the X7R characteristic, a dielectric layer can be made furthermore thinner than in the related art.
As an electronic device according to the invention is not particularly limited. A multilayer ceramic capacitor, a piezoelectric element, a chip inductor, a chip varistor, a chip thermister, a chip resistance and other surface mounted chip electronic devices can be mentioned as examples.